Conversion coated aluminum conductor and method for preparation thereof

ABSTRACT

Preparation of aluminum conductor material and insulated multistrand cable made therefrom by zinc treatment of aluminum followed by chromate conversion coating of the thus treated aluminum and forming same into the insulated cable.

United States Patent Walter G. Zelley Lower Burrell, Pa.

Aug. 26, 1969 July 27, 197 I Aluminum Company of America Pittsburgh, Pa.

Inventor Appl, No Filed Patented Assignee CONVERSION COATED ALUMINUM CONDUCTOR AND METHOD FOR PREPARATION THEREOF 8 Claims, 2 Drawing Figs. U.S.Cl 174/119 R, 29/19l.6,148/6.2,148/3l.5,174/11O FC, 174/126 C Int. Cl 1101b 7/28 Field 01 Search 174/110.6,

[ 56] References Cited UNlTED STATES PATENTS 3,116,176 12/1963 Fuchs l48/6.2' 3,256,071 6/1966 Singleton 29/1 91 .6 FOREIGN PATENTS 217,784 10/1958 Australia 174/126 (.2) 921,453 3/1963 Great Britain 174/1 10 (.6)

OTHER REFERENCES Bellino, How To Select A Multiconductor Cable ln Insulation, 12/67 pp. 64- 65 Primary Examiner-E. A. Goldberg Attorney-Abram W. Hatcher ABSTRACT: Preparation of aluminum conductor material and insulated multistrand cable made therefrom by zinc treatment of aluminum followed by chromate conversion coating of the thus treated aluminum and forming same into the insu lated cable.

CONVERSION COATED ALUMINUM CONDUCTOR AND METHOD FOR PREPARATION THEREOF BACKGROUND OF THE INVENTION This invention involves treatment of aluminum so as to decrease the electrical contact resistance and increase the corrosion resistance thereof. More particularly, it involves a multistrand aluminum conductor cable prepared by treatment of aluminum and the formation of a plurality of strands of the treated aluminum into an insulated cable.

U.S. Pat. Nos. 2,569,030 and 3,l56,763 dealt with applying a zincate, zinciferous or zinc immersion coating to aluminum wire. The term aluminum" as used herein and in the appended claims encompasses substantially pure aluminum, for example, commercially pure electrical conductor (EC) grade fluoride radical. The fluoride radical in this instance may be supplied by any appropriate fluorine containing ion, for example, fluotitanate. (TiF W, fluoborate (BF,,), fluosilicate (SiF f, or fluoaluminate (A1F as well as the simple fluoride ion F. The Cr may be supplied in any known appropriate form such as, for example, chromic acid, or a chromate or bichromate. Further information as to types of chroaluminum, and also commercially employed aluminum base 4 alloys in which the elemental metal aluminum is present in amounts of at least 50 percent by weight. The purpose of this zinc treatment was to cause the aluminum to exhibitthe property ofa high electrical conductivity which would be retained over long periods. U.S. Pat. Nos. 2,563,430, 3,007,8l8 and 3,053,691 teach treating of a zinc surface or the surface of some other metal or alloy, such as steel, with a chromate conversion coating, that is, a coating containing Cr0 and sometimes a fluoride radical, to improve the corrosion resistance of the electrical equipment or like products thustreated made therefrom. U.S. Pat. No. 3,404,046 discloses treatment of a zinc or aluminum surface with a chromate conversion coating containing hexavalent chromium, sulfate and zinc ions to give it good corrosion resistance and paint bonding qualities. Despite the teaching of these patents, however, production of aluminum which has a low surface or electrical contact resistance and at the same time possesses the required high corrosion or salt-spray resistance, plus maintenance of the low surface resistance over prolonged periods of use, has not yet been achieved. This has probably been principally due to the fact that the aforementioned U.S. Pat. Nos. 2,569,030 and 3,156,763 have taught the necessityof removing the zinc coating by acid stripping and the fact that it has heretofore been taught that use ofa chromate conversion coating on aluminum would necessarily lower its electrical conductivity and increase its surface resistance to such an extent that it would no longer be useful in preparation of conductor material or multistrand conductor cable. Accordingly, production of corrosion-resistant aluminum of prolonged low resistance which can be formed into a highly efficient conductor represents a highly desirable result.

SUMMARY OF THE INVENTION It is therefore one object of this invention to provide corrosion-resistant aluminum of low surface resistance. Another 1 object is to provide a multistrand conductor cable the aluminum wires of which have both high-corrosion resistance and low surface or electrical contact resistance. A further object is to provide a method for treating aluminum'in such a manner as to impart thereto a combination of lasting low surface resistance and high-corrosion resistance as well as a method for using the resultant treated aluminum in insulated multistrand conductors or the like. Other objects will be apparent from the description and claims which follow.

In its broader aspects my invention involves zinc-coated aluminum with a chromate conversion coating formed thereon. While I do not wish to be bound by any particular theory as to how the chromate conversion coating is formed, it appears that the chromate reacts in some way with at least some of the zinc and/or aluminum so as to form a film. The zinc coating may be applied in a well-known manner, for example, by zinc plating, or by a zinc immersion treatment such as described in the aforementioned U.S. Pat. Nos. 2,569,030 and 3,l56,763. The chromate conversion coating results from treatment of the zinc-coated aluminum by use of a chromate conversion coating solution, preferably one containing CrO and a mate conversion coating solutions which may be used to treat zinc-coated aluminum according to my invention may be found in the aforementioned U.S. Pat. Nos. 3,053,691, 3,404,046 and 2,563,430.

For best results according to my invention the chromate conversion coatingcontains from about 0.05 to about 2.50 g./l. Cr0 'and from about 0.01 to about 0.50 g./I. F. The preferred pH of the solution is from about 2 to about 4 and the preferred temperature of treatment from about 120 to about 200 It is especially desirable that the surface resistance after treatment according to the invention not exceed about 10 microhms. Especially good results are obtained bytreating the zinc-coated aluminum by using a chromate conversion coating solution for from about 15 to about 60 seconds. While this is our preferred treating time it is also possible according to my invention to employ shorter or longer treatment times.

DESCRIPTION OF THE DRAWING an examination of the drawing which forms a part hereof.

' In the drawing, 4 1

FIG. I is a schematic representation of a cross section of a single strand of aluminum wire containing representative zinc and chromate coatings, respectively, from the inside out,'ac' cording to the instant invention.

FIG. 2 is a schematic cross section of an insulated conductor or cable made up of a plurality of strands of aluminum wire coated as in FIG. 1 circumferentially surrounding a center or axial strand, and having an insulation layer surrounding the plurality of strands of aluminum wire externally and circumferentially.

In FIGS. 1 and 2 of the drawing, the aluminum wires 10 are represented as being coated first by a zinc layer 12 and then on the outside by a chromate conversion coating 14. The external insulation of the wires as shown stranded in cable form in FIG. 2 is represented by insulation layer 16.

The zinc and chromate conversion coated aluminum strands such as those shown in FIG. 1 may be fashioned into the insulated aluminum conductor or cable of FIG. 2 by any appropriate known fabrication procedure, and the insulation layer, for example, polytetrafluoroethylene, applied by any appropriate known process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following table provides examples representative of materials which may be treated according to the invention.

In these examples 6061-T6 alloy (Aluminum Association designation) may be used in the form of aluminum aircraft circuitry wire which is treated in the following manner and subsequently wound in strands into a seven-strand conductor insulated with polytetrafluoroethylene.

1. Clean for 1 minute at 160 F. in a borax-pyrophosphate nonetching cleaner.

2. Water rinse.

3. Immerse for 1 minute at F. in the zinc immersion solution of US. Pat. 2,569,030. g. of zinc oxide (Zn0) and 400 g. of caustic soda (Na0H) per liter of bath, the balance being substantially all water] 4. Water rinse.

5. Immerse for l minute at F. in 1 percent by volume of the indicated chromate conversion coating solution.

6. Water rinse.

The following table summarizes the composition, pH, and effectiveness of various concentrations of chromate conver' sion coating solutions used for treating the zinc-coated 6061 alloy samples. The fluoride radical (here F') was added as HF and the Crl); by chromic acid. Substantially the same results are obtained by using NH l'lF or NH,F to supply the fluoride ion. The electrical contact resistance for the various samples in the table was measured with 0.5 inch diameter copper electrodes at 1,000-lb. pressure, using a Kelvin bridge The weight loss figures confirm the desired minimum attack on the zinc film caused by the chromate conversion coating treatment following the zincate treatment step according to my invention. The salt spray resistance was obtained by a procedure conducted substantially as follows:

The selected specimens were subjected to a percent continuous salt spray test in accordance with Method 6061 of Federal Test Method Standard No. 141, with the significant surface inclined approximately 6 from the vertical. Specimens were exposed for 48 hours as required in the electrical industry. After exposure, the specimens were cleaned in running water and blown dry with clean air. When treated according to this invention the specimens were essentially free from any corrosion attack.

While the invention has been described and several practices for the employment thereof have been set forth, it will be obvious to those skilled in the art that many modifications of the invention are possible without departing from the scope thereof.

Having thus described my invention and certain embodiments thereof, 1 claim:

1. Aluminum adapted for conductor usage having as a base coating thereon zinc substantially covered by a chromate conversion coating.

2. Aluminum wire adapted for stranded conductor usage, said wire having as a base coating thereon a zinc film with a chromate conversion coating thereover.

3. A multistrand conductor comprising a plurality of wound strands of the wire of claim 2 surrounded with insulation.

4. The conductor of claim 3 wherein the insulation is polytetrafluoroethylene.

5. A process for decreasing the electrical contact resistance and increasing the corrosion resistance of aluminum which comprises first applying a film to the aluminum by zinc immer- TABLE I.-SURFACE RESISTANCE MEASUREMENTS (Zincated 6061-16 sheet) Concentration of chromate eonversion coating Composition (g./l.) Surface solution (percent Temp. Time resistance Weight loss Example by volume) CrO: F- pH F.) (sec.) (mlcrohms) (percent) 1...... 0.1 0.05 0.01 3.7 160 60 0.5 g./l. CrOa, 0.1 g./l. 48% HF. 1 0.5 g./l. CF03, 0.15 g./l. NH4HF2.

Table 11, which follows, demonstrates the capability of aluminum treated first with a zinc immersion coating and then with a chromate conversion coating according to my invention for retaining a low surface resistance durin longterm usage, as compared to the lack of such capabi ity for the same TABLE II.EFFECT OF R.H. AT 100 F. ON SURFACE 0.5 g./l. CrOa. 0.2 g./l. NHAF. 4 No measured change.

sion treatment and then forming a chromate conversion coating thereover.

6. The process of claim 5 wherein the zinc coating is a zinc immersion coating.

7. The process of claim 5 wherein the chromate conversion coating is formed by a solution containing CrO; and a fluoride radical supplying F.

8. The process of claim 5 wherein the chromate conversion coating is applied from a solution containing from about 0.05 to about 2.50 g./l. CrO and from about 0.01 to about 0.50 g./l. F, at a pH of from about 2 to about 4, at a temperature of from about 120 to 200 F., and for a period of from about l5to about 60 seconds.

RESISTANCE Resistance, Resistance 16 weeks as prepared, exposure, Treatment microhms microhms Deoxidizedoniy...'.. 22 228 Deoxidized plus Zn immersion only..... 1 225 Deoxidized plus Zn immersion plus HNO;

strip......... 16 165 Deoxidlzed plus chromate conversion c-oat- 8 ing 71 Deoxidized plus Zn immersion plus chr0- mate conversion coating. 1 2 Deoxidized plus Zn immersion plus IINO;

plus chromate conversion coating. 4 29 75 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 595 985 Dated July 27 1971 1nventor(s) Walter G. Zelley It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

C01. 2, line 3 Change "12" to C01. 2, line 3 Change "(BFAQ to --(BF4)' L101. 2, line 4 Change "1" to 2, line a Change "13" to E01 2, line 5 Change "F to F [101. 2, line 14 Change "F to F Col. 3, line L Change "F to F J r J 4, line 58 Change "F to F m1 I 4, line 62 Change "F to F' Signed and sealed this 25th day of January 1972.

I ROBERT GOTTSCHALK -PT FLETCHER JR. Bashing Officer Comm1ss1oner of Patents )RM PO-105O (10-69) USCOMM-DC 6OEl76-F'69 

2. Aluminum wire adapted for stranded conductor usage, said wire having as a base coating thereon a zinc film with a chromate conversion coating thereover.
 3. A multistrand conductor comprising a plurality of wound strands of the wire of claim 2 surrounded with insulation.
 4. The conductor of claim 3 wherein the insulation is polytetrafluoroethylene.
 5. A process for decreasing the electrical contact resistanCe and increasing the corrosion resistance of aluminum which comprises first applying a film to the aluminum by zinc immersion treatment and then forming a chromate conversion coating thereover.
 6. The process of claim 5 wherein the zinc coating is a zinc immersion coating.
 7. The process of claim 5 wherein the chromate conversion coating is formed by a solution containing CrO3 and a fluoride radical supplying F.
 8. The process of claim 5 wherein the chromate conversion coating is applied from a solution containing from about 0.05 to about 2.50 g./l. CrO3 and from about 0.01 to about 0.50 g./1. F, at a pH of from about 2 to about 4, at a temperature of from about 120* to 200* F., and for a period of from about 15 to about 60 seconds. 